1. Field of the Invention
The present invention relates to a power cell, or lifting device for displacing masses, driven by a gaseous or liquid pressure medium, and in particular to a hollow body for lifting masses by inflation by gaseous or liquid pressure, the body being plane in the pressure-free state.
2. Description of the Prior Art
Power cells are known which are made of tension-resistant sheets of plastic foil which are combined, for example, by fusion to form a bellows. These known power cells are either designed as circular disks in top view or else the substantially circular top-view shape is formed by alternating concave and convex segments of the rim. If, as in the first design, the power cells are formed by two disk-like plastic foil sheets fused together along their circumference, they offer the advantage of easy machine-finishing but cause problems on account of the stresses in the foil occurring in the various lifting positions. These stresses are due to the operationally induced shortening of the circumference entailing tangential pressure forces, causing kinks at the cell rim beyond a critical height of lift corresponding to the seam segments. The magnitude of the critical height of lift at which the kinks arise, as well as their number and size, depend on the cell diameter, the curvature of the rim and furthermore on the pressure of the medium, the elasticity of the foil-wall and on any kink-stabilization effect of the foil-seam.
Since a meridian shortening of the girth of a free-standing power cell is unavoidable when the inside volume is enlarged, it has been necessary to achieve an appropriate distribution of the foil stresses and, furthermore, to prevent the occurrence of additional stresses by improving the cell construction.
Therefore, power cells were made in conformity with the second design, in which the tangential compression forces arising in the concave rim segments are eliminated at least in part by tangential tensional forces arising at the convex rim segments. At such a concave rim segment, obtained by suitably shaping the foil sheets, tangential forces acting normally to the seam are predominant, whereas tensions prevailing in the plane of the seam are in the convex parts, the peripheral shortenings being very readily absorbed in the concave rim segments by increasing the radii of curvatures. As a consequence, three-dimensional tensional stresses loading the seam relatively strongly occur in the transition zones from the concave to the convex segments.